1. Field of the Invention
This invention relates generally to a fuel nozzle for use in a gas turbine combustor and more specifically to a fuel nozzle having a premix pilot circuit that can be adjusted or modified to meet a desired flow rate.
2. Description of Related Art
The U.S. Government has enacted requirements for lowering pollution emissions from gas turbine combustion engines, especially nitrogen oxide (NOx) and carbon monoxide CO. These emissions are of particular concern for land based gas turbine engines that are used to generate electricity since these types of engines usually operate continuously and therefore emit steady amounts of NOx and CO. A variety of measures have been taken to reduce NOx and CO emissions including the use of catalysts, burning cleaner fuels such as natural gas, and improving combustion system efficiency. One of the more significant enhancements to land based gas turbine combustion technology has been the use of premixing fuel and compressed air prior to combustion. An example of this technology is shown in FIG. 1 and discussed further in U.S. Pat. No. 4,292,801. FIG. 1 shows a dual stage dual mode combustor typically used in a gas turbine engine for generating electricity. Combustor 12 has first stage combustion chamber 25 and a second stage combustion chamber 26 interconnected by a throat region 27, as well as a plurality of diffusion type fuel nozzles 29. Depending on the mode of operation, combustion may occur in first stage combustion chamber 25, second stage combustion chamber 26, or both chambers. When combustion occurs in second chamber 26, the fuel injected from nozzles 29 mixes with air in chamber 25 prior to ignition in second chamber 26. As shown in FIG. 1, an identical fuel nozzle 29 is positioned proximate throat region 27 to aid in supporting combustion for second chamber 26. While the overall premixing effect in first chamber 25 serves to reduce NOx and CO emissions from this type combustor, further enhancements have been made to the centermost fuel nozzle since fuel and air from this fuel nozzle undergo minimal mixing prior to combustion.
A combined diffusion and premix fuel nozzle replaced the diffusion type fuel nozzle shown proximate throat region 27 in FIG. 1. Although an improvement, this nozzle still contained a diffusion fuel circuit that contributed to elevated levels of NOx and CO emissions. As a result, this fuel nozzle was modified such that all fuel that was injected into a combustor was premixed with compressed air prior to combustion to create a more homogeneous fuel/air mixture that would burn more completely and thereby result in lower emissions. This improved fully premixed fuel nozzle is shown in FIG. 2 and discussed further in U.S. Pat. No. 6,446,439. Fuel nozzle 50 contains a generally annular premix nozzle 51 having a plurality of injector holes 52 and a premix pilot nozzle 53 with a plurality of feed holes 54. In this embodiment, fuel enters a premix passage 55 from premix pilot nozzle 53 and mixes with air from air flow channels 56 to form a premixture. Fuel nozzle 50 is typically utilized along the centerline of a combustor similar to that shown in FIG. 1 and aids combustion in second chamber 26.
Although the fully premixed fuel nozzle disclosed in FIG. 2 provides a more homogeneous fuel/air mixture prior to combustion than prior art fuel nozzles, disadvantages to the fully premixed fuel nozzle have been discovered, specifically relating to premix pilot nozzle 53. Depending on the base load operating conditions, compressor air flow, and other factors, the amount of fuel required to be injected through holes 52 and 54 will vary from engine to engine, and therefore, producing a common fuel nozzle for different engines is not possible. This is especially a disadvantage with respect to premix pilot nozzle 53, for which feed holes 54 must be machined prior to assembly of fuel nozzle 50, since feed holes 54 are inaccessible once premix pilot nozzle 53 is installed in fuel nozzle 50. Therefore, it is necessary to know fuel flow requirements of the fuel nozzle for each engine before fuel nozzle assembly 50 is assembled. As a result, this prohibits the storage of completed fuel nozzle assemblies for a wide variety of engines. Furthermore, having individual or “custom” flowing fuel nozzle designs prevents the engine operator from interchanging fuel nozzles between different flowing engines. In addition, from the manufacturer's perspective, it would be advantageous to have a uniform design assembled, which can be shipped to an engine operator on short notice. Therefore, what is desired, and is disclosed in the present invention, is a fully premixed fuel nozzle for a combustor, which can be fine-tuned through an interchangeable or adjustable premix pilot nozzle. A variety of alternate embodiments of the present invention are disclosed in detail.